Get rid of VertexFormatType. The enum VertexFormatType is redundant with angle::FormatID. The Vulkan back end has already eschewed VertexFormatType, this change updates the D3D back ends. BUG=angleproject:2531 Change-Id: I67ea2203ca80be828f4c757a37810fe67a279364 Reviewed-on: https://chromium-review.googlesource.com/c/1263899 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Frank Henigman <fjhenigman@chromium.org>